No offense, but what exactly do you plan to do? The MIDI authors can't sue him, since the music in question doesn't belong to them either. I read that leinatorz guy's log, and his idea of copyright law is WAY off base.
I'm afraid it doesn't work that way. We technically don't own the arrangements hosted here on OCR, R:TS, and VGMix. In order to sell them would require licensing from the original owners (usually the game companies, but in same cases the composers) which is difficult to obtain. In fact if SquareSoft or Nintendo wanted to sue us, they could. And they would probably win. But they won't, for the same reason they don't sue fan artists-- mostly because it would greatly alienate their fanbase and would be very bad for publicity should the news spread. They know we do it out of love.
Now don't get me wrong, I do not condone what this guy is accused of in any way (though it would be hypocritical of me not to note that I have been guilty of it in the past... the very distant past). At R:TS we are much more lax about quality issues and standards, but even so, we simply will NOT allow MIDI rips. The same holds true for OCR and VGMix. I'm not aprised of the details of the issue, but unfortunately it seems that this guy is operating in a gray area. His revenue apparently comes only from donations, thus he's not directly selling the songs. Therefore you can't really make the case that he's selling works that aren't his own for profit. There's not much you can do in this case but call the guy a big flaming stupid head, shun him, and go about your business.